The development of Face Gears for high power transmission is a relatively recent phenomenon. Historically, the transmission of power through a Face Gear set was limited to relatively low levels because of two factors: the tooth profile of the mating gears was generated by shaper cutting and, although as very acceptable tooth profile could be generated, the tooth produced by the shaping operation did not have a hardened surface. The tooth profile produced by the shaping operation required that the resulting Face Gear set be kept in almost perfect alignment.
Any operation performed on the Face Gear set to harden the surface of the shaped teeth tended to distort the shape of the Face Gear set during the hardening operation.
The current method of manufacture of Face Gears was developed by the Fellows Corporation using a gear shaper apparatus and the finished product is useful for the transmission of power for low power applications.
Recently however, development has been undertaken by McDonnell Douglas Helicopter Systems supported by NASA Lewis Research Center with regard to designing and developing Face Gears for use in high power applications (Ref. NASA Technical Memorandum 106101/AVSCOM Technical Report 92-C-009).
The applicant, herein, has successfully developed the manufacturing practices and the associated equipment required to produce Face Gear sets for high power transmission applications.
The Face Gear method of manufacture developed by the Fellows Corporation of shaping the gear teeth, is a metal cutting process, which can only be applied to materials with suitable hardness and metal cutting characteristics. If the material is too hard, the shaper tool will not cut effectively. This shaping process can only be used effectively for finish cutting Face Gear teeth from metals suitable for low power applications. This process does not give the accuracy and surface finish required for higher power applications.
Gear blanks are roughly machined as in the prior art, to produce toothed wheels wherein the gears produced have slightly enlarged teeth which makes allowance for a subsequent grinding operation. The Fellows shaper method is quite acceptable for the production of gears from blanks in this operation.
The gear (now having enlarged shaped teeth) is thence subjected to a heat treatment operation to increase the surface hardness of the gear teeth. During this operation, the rough cut gear will usually undergo some physical distortion which occurs during the heat treatment operation. The excessive material deliberately left on the gear teeth in the gear shaping operation, will be sufficient to allow a subsequently distorted gear to be restored to its required shape by a grinding operation.
The heat treated gear is now ground to the final shape and accuracy having the desired tooth profile. At this stage, the resulting gear has a hardened tooth on a gear platform which is quite stable because of the stress relieving operation. Grinding is the only known method that will produce the accuracy and surface finish required for high power transmissibility applications.
The surface grinding operation is a continuous operation with the grinding wheel and the face gear constantly rotating and moving such that the grinding wheel moves across the face of the rotating face gear in a controlled fashion. The grinding wheel has a surface which is commonly referred to as a xe2x80x9cwormxe2x80x9d and in grinding a face gear, the surface of the grinding wheel is being constantly eroded by its constant engagement with the hardened metal surface of the previously formed teeth on the face gear. After deposits of the grinding debris, both from the erosion of the grinding wheel and from the material removed from the gear teeth during the grinding operation tend to be redeposited on the surface of the grinding wheel during the grinding operation. A suitable dressing wheel mounted on the grinding machine periodically restores the worm profile to its proper configuration.
In the manufacture of spur gears, the movement of a diamond dressing disc used to restore the profile of the spur gear grinding wheel must be controlled in both X and Y axes.
This application requires that the movement of a suitable dresser disc be controlled in both the X and Y axes as previously for spur gears, but also the disc must be controlled for movement in a pivot axis (designated the xe2x80x9cAxe2x80x9d axis) to produce the desired grinding wheel worm profile to properly shape the teeth on the Face Gears being ground.
In addition to having the dresser tool move in the X, Y and A axes, the tool must be capable of manual adjustment in two additional axes.
Gear tooth grinding of spur gears is performed by the coordinated rotation of the grinding wheel and a gear blank so that the grinding wheel worm engages the gear blank in a constant meshing operation during the grinding operation. The area of engagements of the worm of the grinding wheel with the rotating spur gear is changed by moving the spur gear rectilinearly in its axial direction during a grinding operation to complete the tooth forming operation.
In face gear grinding operations, the axis rotation of the grinding wheel relative to the face gear is significantly different. The reason for this is that for face gear grinding operations, the grinding wheel must move the complete distance along the length of the teeth of the face gear and be parallel to the plane of the surface of the gear face in which the face gear teeth are being formed.
In grinding face gears, the angle of the teeth generated in the gear face may vary widely with respect to the rotational axis of the face gear, thus grinding of the teeth of a face gear presents a substantial challenge.